Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumene. The second step is oxidation, preferably aerobic oxidation, of the cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide into equimolar amounts of phenol and acetone, a co-product.
It is also known that phenol and cyclohexanone can be co-produced by a variation of the Hock process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is disclosed in U.S. Pat. No. 6,037,513, which discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product.
There are, however, a number of problems associated with producing phenol via cyclohexylbenzene rather than the cumene-based Hock process. Firstly, oxidation of cyclohexylbenzene to cyclohexylbenzene hydroperoxide is much more difficult than oxidation of cumene and requires elevated temperatures and the use of a catalyst, generally a cyclic imide, such as N-hydroxyphthalimide (NHPI), to achieve acceptable rates of conversion. Additionally, it was generally thought that the cyclic imide catalysts needed to be removed from the oxidation products prior to the cleavage step because they may cause problems in downstream separation processes and affect the quality of the final products.
For example, PCT Patent Publication WO2010/042261 discloses that unreacted cyclic imide catalyst can act as a poison to the downstream cleavage catalyst (e.g., mixed metal oxide). Thus, it will normally be desirable to treat the effluent from the oxidation process to reduce the level of unreacted cyclic imide prior to passage of the effluent to the cleavage step. Generally, the effluent is treated so as to reduce the level of the imide in the organic phase to less than 100 ppm, such as less than 50 ppm, for example less than 10 ppm, by weight of the organic phase.
Moreover, PCT Patent Publication WO2010/098916 also discloses that it will normally be desirable to treat the effluent stream from the oxidation process to remove at least part of the cyclic imide of the first catalyst prior to passage of the effluent stream to the cleavage of the hydroperoxide. In a preferred embodiment, the cyclic imide is removed in a separate vessel that is downstream of the oxidation reactor and upstream of the cleavage reactor. PCT Publication Nos. WO2009/025939; WO 2009/058527; and WO2011/041801 also disclose methods for removing cyclic imide catalysts prior to cleavage reactions.
According to the present invention, it has now been found that the presence of cyclic imide oxidation catalyst in the cleavage feed does not affect selectivity in the cleavage reaction when using certain cleavage catalysts (e.g., sulfuric acid and/or solid acids). This discovery eliminates the need to remove the cyclic imide prior to the cleavage step, resulting in the savings of both capital and operating costs.